In the following, the following designations are used:
“Mid-plane”: a plane containing the rotation axis of the tire.
“Equatorial plane”: the plane passing through the middle of the tread surface of the tire and perpendicular to the rotation axis of the tire.
“Radial direction”: a direction perpendicular to the rotation axis of the tire.
“Axial direction”: a direction parallel to the rotation axis of the tire.
“Circumferential direction”: a direction perpendicular to a mid-plane.
“Radial distance”: a distance measured perpendicularly to the rotation axis of the tire and from the rotation axis of the tire.
“Axial distance”: a distance measured parallel to the rotation axis of the tire and from the equatorial plane.
“Radially”: in a radial direction.
“Axially”: in an axial direction.
“Radially inner, respectively radially outer”: of which the radial distance is lesser, respectively greater.
“Axially inner, respectively axially outer”: of which the axial distance is lesser, respectively greater.
A tire comprises a tread designed to come into contact with the ground, two sidewalls extending radially inwards from the ends of the tread, and two beads extending the sidewalls radially inwards and providing the mechanical connection between the tire and the rim on which it is mounted.
A radial tire comprises more particularly a reinforcement element, comprising a crown reinforcement, radially inside the tread, and a carcass reinforcement, radially inside the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of the civil engineering type usually comprises at least one layer of carcass reinforcement consisting of metal reinforcement elements coated with a coating polymer material. The metal reinforcement elements of the carcass reinforcement layer are substantially parallel with one another and have a substantially radial direction, that is to say that they make, with the circumferential direction, an angle of between 85° and 95°.
The carcass reinforcement layer comprises a main carcass reinforcement portion, connecting the two beads together and winding in each bead, from the inside to the outside of the tire, around a bead wire core, in order to form a carcass reinforcement upturn extending radially outwards to a carcass reinforcement upturn end and comprising two respectively axially inner and axially outer carcass reinforcement upturn faces.
The bead wire core usually consists of a circumferential reinforcement element most frequently made of metal surrounded by at least one, nonexhaustively polymer or textile, material. The winding of the carcass reinforcement layer around the bead wire core, from the inside to the outside of the tire, and forming a carcass reinforcement upturn extending radially outwards, anchors the carcass reinforcement layer to the bead wire core of the bead.
The radial positioning of the carcass reinforcement upturn end is characterized by the carcass reinforcement upturn height which is the radial distance between the carcass reinforcement upturn end and the radially innermost point of the bead wire core. The carcass reinforcement upturn height determines the anchoring of the carcass reinforcement upturn in the polymer blends in contact respectively with the axially inner and axially outer carcass reinforcement upturn faces. The carcass reinforcement upturn height may be defined with the aid of a ratio relative to the radial distance between the radially outermost point of the tread and the radially innermost point of the bead.
“Axially inner carcass reinforcement upturn face” means the carcass reinforcement upturn face of which the external normal at any point of the said face has an axial component directed towards the inside of the tire. “Axially outer carcass reinforcement upturn face” means the carcass reinforcement upturn face of which the outer normal at any point of the said face has an axial component directed towards the outside of the tire.
The axially inner carcass reinforcement upturn face is in contact with a filling element radially extending the bead wire core outwards. The filling element has, in any mid-plane, a substantially triangular section and consists of at least one polymer filling material. The filling element may consist of a stack in the radial direction of at least two polymer filling materials in contact along a contact surface cutting any mid-plane along a meridian line. The filling element axially separates the main portion of carcass reinforcement and the carcass reinforcement upturn.
The axially outer carcass reinforcement upturn face is at least partly in contact with a stuffing element consisting of a polymer stuffing material. The stuffing element is axially inside the sidewall and a protective element radially extending the sidewall inwards, the sidewall and the protective element respectively consisting of a sidewall polymer blend and at least one protective polymer blend.
A polymer material, after curing, is characterized mechanically by characteristics of tensile stress-deformation determined by tension tests. These tension tests are carried out by those skilled in the art on a test specimen, according to a known method, for example according to international standard ISO 37, and in normal temperature conditions (23+ or −2° C.) and hygrometry conditions (50+ or −5% relative humidity), defined by international standard ISO 471. The modulus of elasticity at 10% elongation of a polymer blend, expressed in mega pascals (MPa), refers to the tensile stress measured for a 10% elongation of the test specimen.
A polymer material, after curing, is also characterized mechanically by its hardness. Hardness is notably defined by the Shore A hardness determined according to the standard ASTM D 2240-86.
When the vehicle is running, the tire, mounted on its rim, inflated and squashed under the weight of the vehicle, is subjected to flexing cycles, in particular at its beads and its sidewalls.
The flexing cycles cause variations of curvature combined with variations of tension of the metal reinforcement elements of the main carcass reinforcement portion and of the carcass reinforcement upturn.
For a tire with a carcass reinforcement upturn known as high, that is to say for which the carcass reinforcement height is at least equal to 0.3 times the radial distance between the radially outermost point of the tread and the radially innermost point of the bead, the flexing cycles in the sidewall cause the breakage of the metal reinforcement elements of the carcass reinforcement upturn portion situated in the flexing zone of the sidewall, capable of causing a deterioration of the tire over time requiring its replacement.
For a tire with a carcass reinforcement upturn known as low, that is to say for which the carcass reinforcement height is at most equal to 0.3 times the radial distance between the radially outermost point of the tread and the radially innermost point of the bead, the flexing cycles in the bead cause cracking of the polymer blends situated in the vicinity of the carcass reinforcement upturn end in a zone of high mechanical flexing and shearing stresses, capable of causing a deterioration of the tire over time requiring its replacement. This cracking phenomenon at the end of the carcass reinforcement upturn also exists, but to a lesser degree, in the case of a high carcass reinforcement upturn.
In the case of a tire with a high carcass reinforcement upturn, in order to prevent the problem of breakage of the metal reinforcement elements of the carcass reinforcement upturn portion, situated in the zone of flexing of the sidewall, those skilled in the art have proposed to reduce the height of the carcass reinforcement upturn in order to achieve a low carcass reinforcement upturn which is nevertheless sensitive to the cracking of the polymer blends, situated in the vicinity of the carcass reinforcement upturn end.
In the case of a tire with a low carcass reinforcement upturn, document EP 0736400 describes a solution for solving the problem of cracking of the polymer blends, situated in the vicinity of the carcass reinforcement upturn end, consisting in coating the carcass reinforcement upturn end with a polymer material absorbing the deformations of the polymer blends that are present in this zone.